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A Peek Inside the Canon 24-70 f/4 IS

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If we knew what we were doing it wouldn’t be called research, would it?  Albert Einstein

Of course we had to do it. One of them was soft, you see, and we were really curious about how that Macro mode switch worked and  . . . . OK, we just wanted to. Actually, we've found the new 24-70 f/4 IS is a most interesting lens inside.

So as usual, Aaron is doing the disassembly while I'm doing the photography and writing.

Up Front First

There's a plastic cosmetic ring that pops out.

 

Which uncovers the 6 screws holding the front group in place. Like the 24-70 f/2.8 Mk II (and unlike the Mk I) there are no adjusting tilt or centering screws on the front element.

With those out the front group is comes right out. Hopefully this will keep replacement costs for a scratched front element low.

Underneath the front element, the distal barrel / filter ring is held on by an arrangement we've never encountered before.  Each of the 6 screws mounts through a metal sleeve, compressing a spring as it tightens the front barrel in place.

 

As is often the case when we see something new, we aren't certain what the purpose is. Our first thought was shock absorbing, but the screws are tightened down pretty well so there's no spring action. Our second thought was front element tilt (by tilting the whole barrel) but we see no shims or washers that would (we think) be used if that were the purpose. On the other hand, one of the lenses we received that was soft at 70mm had a couple of these screws a half turn loose and was much better after they were tightened.

 

Anyway, if you take these screws out, the distal barrel / filter ring comes right off. (Since we did not know their purpose, we considered not messing with them. For about 2 seconds.)

Again, this should make a simple repair when a filter ring breaks. Just for demonstration purposes, a couple of shots with the internal barrel zoomed out to 70mm and back to 24mm (retracted).

We left the tube/spring assemblies in place with the screws removed to show them again. Maybe one of you can explain what they are. (That's a subtle hint to any Canon techs who are visiting. We haven't outed anyone else who told us stuff we aren't supposed to know. We won't out you either -- we'll just say we figured it out ourselves, OK?)

BTW - this second element is one of the focusing elements. Notice the focus key at 11 o'clock in the picture above. OK, that's all there is to see from the front of the lens, so have a cup of coffee while we put this part back together and move to the back side.

 The Back Side is More Interesting

The back comes off in the usual way, showing us the main PCB. Other than a rather amazing 8 flexes that have to be unplugged it's pretty routine.

 

The PCB has a bit more circuitry than older lenses, laid out in Canon's usual efficient and orderly fashion. There are a number of flush-mounted components, many covered by heavy electrical shielding tape.

At this point we come to another thing we've never really seen before, at least not to this degree. The single rear element is sort of on stilts, projecting out to the back of the lens all by it's lonesome - at least with the lens racked out to 24mm (more on that later). That's a big gap between the rear element and the next element forward.

There are a healthy number of screws (8 if I recall correctly) holding the rear barrel in place. Once it's removed we again see Canon's usual tightly organized flex runs to the switches in the barrel. I know it doesn't make a bit of difference in what kind of pictures it takes, but it makes me feel comfortable knowing some engineer has taken the time to map out each flex run, etc. Wish some other manufacturers did.

One thing I was very interested to see was the zoom lock-macro switch and how it worked since this is a first. The good news is the switch is easily removed or changed from outside with no lens disassembly -- it's a simple mechanical switch with a plastic tab that slips under the zoom ring fully to lock, and must be pulled back against a spring to allow Macro work.

I mention it's good that it's accessible because I'm going to be replacing a lot of these. It probably won't affect any of you who buy the lens, but I guarantee renters who aren't familiar with the macro switch are going to force it (which I tested and it's easy to do) and eventually break this little plastic tab off. That won't have any bad effects other than letting you accidently move into macro mode, which I doubt will be a problem for anyone, really.

A few more screws and the motor - focusing assembly unit comes off. I really like how Canon is putting this as a removable unit in their newer lenses. It makes disassembly and repair much simpler. Maybe that's one of the reasons Canon repair prices are more reasonable.

With the motor unit off the internal optical assembly containing all lens elements, helicoids, etc. is left as a group.

This gives us an opportunity for a clearer look at that rear element. At 24mm focal length it sits up there looking rather lonely and vulnerable.

As you zoom the lens out, though, it moves forward at a brisk pace. At 70mm it's tucked inside the barrel, right up against the next optical group.

The most interesting thing is when the switch is flipped and the lens moves into macro mode, the rear element and next group move even further forward, moving as a single group.

From the outside of the optical group, you can see the very long helicoid groove that causes this large amount of travel. For those who don't look at helicoid grooves often, they are usually simple, smooth curves. If you're a weird photo geek, you can trace them out on paper and reproduce exactly how the elements move in relation to each other (I don't know anyone who would bother to do weird stuff like that, of course).

 

This groove is unique. I've never seen one like it. You can see the notch in the groove (red arrow) that begins macro mode, with a different curve above and below that notch. It shouldn't be surprising, I guess, since in macro mode infinity focus is given up and the elements are used to bring much nearer focus than would otherwise be possible. But I've never seen anything quite like it. Brilliant idea and the engineering and design mathematics involved must be most formidable. I haven't found it, but I'm certain there's a patent out there somewhere for this. It really is like having two different lenses in one.

Finally, one of the things we tend to look at is where the lens is optically adjusted. It probably shouldn't be surprising that such a complex lens has a complex adjustment setup. As is usual for Canon (and one of the things all PhotoGeeks love) most of the adjustments are done through oblate collars, allowing very fine tilt and centering movements, unlike the shims used in many other brands.

Two of the rear groups have white nylon adjustment collars. I'm pleased to note they are thick, heavy-duty collars so they should be less likely to break or wear with use. The upper (leftmost in the picture) collar is very obviously oblate so it will allow large movements, while the lower (rightmost) is a more subtle adjustment.

Deeper in the optical barrels are two forward elements that are adjusted by brass-collared screws. I've marked them with red arrows since they're more difficult to see.

That's at least 4 adjustable groups in this lens, which is more than most lenses have. Again, it makes sense - this is really two lenses in one. I'm guessing some of the adjustments will be largely effective at only macro ranges, while others will have more effect at normal shooting ranges.

 Conclusion:

Well, this was one of our longer teardown posts and I assume we've lost everyone but the true geeks by now. But the Canon 24-70 f/4 IS is a truly interesting design, with some things that we've never seen before.

Some worry me a tiny bit. I'm certain the zoom-lock/macro switch is going to break some, although it doesn't appear that will have any major effect.

The large number of adjustable elements is a bit concerning, too. In theory, at least, that's more adjustments that could contribute to sample variation. On the other hand, Canon's been a long time developing this lens and they engineer lenses more logically than anyone else. I suspect there's a nice thick document I'll never see that tells techs exactly which of those adjustments controls which aspects of the lens' optics.

In some other lenses, where one element is used to adjust several variables the adjustments can be ridiculously finicky to make. It might well be that having more adjustable spots will make the adjustments more straightforward and logical, reducing sample variation. Only time will tell on that one.

 

Roger Cicala and Aaron Closz

Lensrentals.com

January, 2013

31 Responses to “A Peek Inside the Canon 24-70 f/4 IS”

Ralph Conway said:

Thank you very much again, Roger (and Aaron of course). Very interesting to read and see what is going on inside this lens. So now I am looking forward for some sample images, done with it and some feedback about AF accuracy and speed.

Happy weekend ;-)

Ralph

Mister O said:

As usual a great effort and wonderful insight. Thank you! You've piqued my interest, though- now that you've had a peek inside, where's the peak?

Roger Whitehead said:

> A Peak Inside the Canon 24-70 f/4 IS

I still can't see the mountain, or did you mean "A peek"?

Roger

LensRentals Employee

Roger Cicala said:

hehehehehe I have been to the mountaintop. And misspelled it.

Jim Harris said:

Is that all it take for me to qualify as a true geek -- getting to the end? :) Terrific teardown, Roger. I'm impressed by the design aspects and how they differ and parallel the 24-70/2.8 II. Some really new thinking at Canon it seems. I'm curious to how complex you feel these adjustment screws are to the design of the 16-35L II? I found problems with new version about 6 months in, after checking thoroughly. It took three trips to CPS to get major improvement, following incremental improvements at trip 1 & 2. Just wonder if this 24-70/4 seems to have more variables than the 16-35II. Thanks again!

Bart said:

The two reasons I can think of for the spring-screw arrangement are:

* pre-tension of the screw fastener. This reduces dynamic load on the screw, and as such lowers chances of failure due to metal fatigue. I can't judge to what extend those particular screws would be subject to that though.

* aligned with the above point, it's a way to prevent screws from vibrating loose. Are you sure there's not two-stroke generator secretly attached to that lens, for powered focusing?

CarVac said:

You keep noting all the great design in Canon lenses, which I can clearly see. Care to show an example of bad design? From a major brand (Nikon?) perhaps?

Regarding those zoom tracks, it looks like they're all kinked in funny ways. Maybe this is why these new lenses are a) so expensive to make and b) perform rather well.

...One thing I want to know, just in case you've taken one apart. I once rented a 100-400, and the focus ring (and autofocus drive motor) seems to move elements at the same rate at all zoom settings (with a larger range at full telephoto). Thus, it takes more rotation (or time, when autofocusing) at full telephoto to rack focus end-to-end. However, the focus scale seems to have a continuously variable linkage so that no matter what zoom setting, the varying-width range (whether short or long) of focus group motion corresponds perfectly with a constant range of focus scale motion.

How on earth does this work? It's basically some sort of continuously-variable transmission with the ratio linked to the zoom position... it blew my mind when I started thinking about it.

Hasan Karagulmez said:

Amazing work, Aaron and Roger!

Nice to see these lenses being taken apart :)
I also like to see how manufacturers take a different route designing their lenses throughout the years.

One question, out of interest (not fanboyism!): which manufacturers would you say *are* less "clean" in their design, regarding mapping out flex cables and such, and have you got samples?

alek said:

That was even more awesome than usual Roger - great job.

Some real interesting (and apparently well thought out) engineering went into the design of this lens with consideration for manufacturing/repair.

LensRentals Employee

Roger Cicala said:

Bart, I like those ideas :-)

Maybe it's a tensioner to keep them from overtightening and stripping the plastic threads?

LensRentals Employee

Roger Cicala said:

Hasan,

Nikon and Sony's lenses are very similar and both are horrid to work on. Rather than a nice single circuit board like Canon, they have several square ones wrapped in a triangle around the central barrel (one of the reason Nikon lenses are wider, I suspect). Also lots of long wire runs with soldered ends rather than flexes, etc.

Almost everyone else tends to use shims and spacers rather than adjustable screws, too.

Feng Chun said:

it looks like clearly the Macro mode contribute quite much to the price hijack...

Dr Croubie said:

I'm still waiting for a teardown of a Lensbaby...

KimH said:

Hi Roger & Co

This is SOOO cool - thanks for sharing.

The math behind creating a good lens - especially zooms is immense.

On top comes all the bells and whistles IS, AF, Macro, ...

I am sure you are already working this for a future article, but i am keen to hear if one of the "flexes" is connected to a Focus Sensor.

Pete Johnson said:

Roger,I think you and Bart have it on the money. Springs will allow the screws to stay in place with very low torque values.They will also allow the screws to stay in place at less than fully tight (think carburetor idle mixture screws.

Nate said:

This reminds me of the Vivitar Series 1 70-210mm f/3.5 lenses. They also use a mechanical switch to engage a similar macro mode. When in macro mode, the push-pull zoom is re-purposed as the focus/magnification control.

Great article as usual!

Trenton said:

Interesting design. However, I find it disturbing that everyone seems to be concentrating on f/4 lenses lately. We need new generation of f/2.8 wideangle zooms, and we (event photographers) need it badly. Canon users are lucky, EF 16-35/2.8L II focuses super fast in low light. But for Nikon, we have long discontinued 17-35/2.8 and... Blank. Nothing to replace it with, only stupid 16-35/4G. I am not talking about sharpness or CA, I am talking about low light and low contrast AF. No f/4 lens, even on a camera with the most superb AF sensor, will ever lock focus in low light faster and more reliable than f/2.8 does.

Pat Farrell said:

Great tear down. Does Canon sell you repair parts? You mention a couple of times that you expect the macro switch to break. Can you get new ones? or do you need to take a good one and have a friend with a MakerBot setup a way to 3D print some spares?

Charlie Morrison said:

Can we see a Nikon teardown at some point ?

LensRentals Employee

Roger Cicala said:

Pat, Canon, unlike some other companies, will happily sell repair parts to anyone who asks.

LensRentals Employee

Roger Cicala said:

Charlie,

Definitely. I keep talking about 'well organized' and 'well engineered' but realize people don't have much to compare it to. We'll try do some other brands soon.

Ed M. said:

Great work, Aaron and Roger. Much better than that hack job I did on the rented 400/2.8! The six screws and springs - my thought is that arrangement helps to keeping the distal barrel/filter ring centered during installation. I don't know how tight the fit is, but 'twould be a bear to cock that while installing. Pleasantly surprising to know that Canon sells repair parts.

John R. said:

Always enjoy your articles.

Since you noticed that the one lens that was soft had some of the screws a half turn loose, could the screws/springs be used to align the tilt of the front group of elements in relation to the rest of the lens?

vikta said:

A no nonsence break down, well done. I have a copy of this lens and it's ultra sharp as you charts suggest. just haven't tested the macro mode thoroughly. Thansk for all the added info and hard work.

Shimon said:

Nicely done. Thanks for posting.

I took apart a Nikon 20/2.8 lens a couple of years ago to clean oil off the blades and posted an article with photos here for anyone interested: http://shimonmor.com/TEMP/20mmlensrepair.html

LensRentals Employee

Roger Cicala said:

Hi Todd,

That makes perfect sense and would be useful in a part that has to function as a shock absorber every so often.

Kevin Purcell said:

Roger:

I note that there are only 5 spring loaded screws laid out in a hexagonal pattern. It would have been easy to add the sixth so like the "dog not barking" in Sherlock Holmes I think there is a clue there.

It gives a single fixed point about which one screw controls the tilt in one direction and two pairs of two control the distance and tilt in the orthogonal direction. A measurement system in the top would allow them to get the tilt rapidly in two directions.

I wonder if Nikon is changing it's ideas about production engineering and easy of repair with the Nikon 1 cameras. They massivley reduced the number of parts in the bodies (from the DSLR part count).

Can the same can be said of the Nikon 1 lenses too. I've love to see a teardown of a Nikon 1 lens and compare that to a current Nikon full frame lens and an AF-S DX lens. One also wonders about the 35mm f/1.8 DX lens: inexpensive but good so was it designed for easier manufacture than previous AF-S lenses?

Plenty of things to pull apart :-)

Rahman said:

Another lens p*rn, Sir! Haha.

But this seriously prompted me to strip my pop's old Sigma 28-200 UC. Its front element is easily popped off, but now the lens isn't as sharp (it was never really sharp to begin with).

What tools do you recommend to start such teardown, apart from a set of precision screwdriver and a pinch of sanity?

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